Recovery of nitrogen from reject waters of biogas process (BioNH4)

Nitrogen is an important nutrient in agriculture in addition to phosphorous. Most of the nitrogen is currently produced via Haber-Bosch method, which utilized fossil fuels and produces ca. 1.2% of the greenhouse gas emissions globally. Various municipal and agricultural waste and side streams include nutrients that should be more efficiently recovered and recycled. For example, there is ca. 3 700 tons of nitrogen in municipal and industrial wastewater sludges in Finland. Often, municipal wastewater sludges are processed in a biogas process that generates digestate and the liquid fraction of the digestate, i.e. reject water, contains large concentrations of ammonium-nitrogen. Recovering nitrogen from the reject water would enable (i) production of renewable nitrogen-rich fertilizers, (ii) decreasing greenhouse gas emissions, and (iii) decreasing the energy and chemical consumption in the wastewater treatment plant.

In this project, we are developing and optimizing a bench scale bioelectrochemical process (BES) that concentrates ammonium-nitrogen from reject water. The ammonium-nitrogen is concentrated to the cathode, where the pH increases due to reduction reactions converting liquid ammonium to gaseous ammonia. The concentrated ammonium-nitrogen is separated from the cathode with membrane contactor that recovers ammonia and absorbs it into e.g. sulphuric acid. Thus, producing a commercial product, ammoniumsulfate.

The membrane contactor has been earlier studied from more dilute liquid fractions that has smaller salt concentrations, the efficiency of membrane contactor in more concentrated waters produced with BES is optimised in laboratory scale before integrating to the bench scale BES reactor.